Single-stage fiber amplifiers are limited in their ability to amplify a source signal. To further amplify a source signal, the outputs of multiple fiber amplifiers can be joined together to produce a higher power output signal. To achieve high amplification powers, the amplifiers are further arranged into multiple stages, with the output of a first stage fiber amplifier being the input to a higher power second stage fiber amplifier. Phase locking the fiber amplifiers prevents a loss in power output due to destructive interference between the various output signals.
One method of phase locking the multiple fiber amplifiers is described in U.S. Pat. No. 6,400,871 to Minden. Minden describes a method of coherently phase combining multiple fiber amplifiers by modulating the pump current of the multiple fiber amplifiers. Modulating the pump current of a fiber amplifier changes the gain of the fiber amplifier and induces a change in the phase of the output. A detector receives a portion of the output from each fiber amplifier and detects the change which is then used in a feedback loop to adjust the phases of each of the fiber amplifiers in order to phase lock the fiber amplifiers together.
The method described by Minden works so long as there is a detectable change that correlates to a corresponding change in phase. To achieve high power outputs, multiple stage fiber amplifiers are used. However, when the gain of the first stage is modulated in order to induce a change in phase, the modulating of the gain also modulates the output power that is being input into the second stage. The modulation in the output power changes the gain of the second stage, which causes an opposite change in phase in the second stage fiber amplifier. At high powers, the change in phase induced by modulating the pump current in the first stage is cancelled by the opposite change in phase induced by the changes to the gain of the second stage.
Also, particularly at high power amplification levels, system noise can cause amplified spontaneous emission (ASE), or unwanted lasing to spontaneously occur, especially when the source signal is a pulsed source signal.